Abstract
Mg-air batteries are considered important power sources especially for vehicles because of their remarkable high theoretical energy density and low cost. Mg-air batteries have a theoretical energy density of 6800Wh/kg, specific capacity of 2200 Ah/kg, and a theoretical cell voltage of 3.1V. The theoretical performance of Mg-air battery shows its comparability with Li-ion battery and can also considered better due to its low toxicity and easy availability. But, Mg-air battery has some limiting factors like low operating voltage, low columbic efficiency due to parasitic reactions such as hydrogen evolution, sluggish oxygen reduction reaction kinetics at air cathode. Our project will focus on the corrosion mechanism of magnesium and Mg-RE alloys in different electrolytes with particular reference to HER and battery discharge. The polarization behavior of different Mg-Rare earth alloys were evaluated in different heat-treated conditions and different surface conditions along with volumetric determination of hydrogen gas evolved during cathodic and anodic polarization.
Mg-RE alloy ZE10A in three different heat treatment condition (a) as received, (b) heat treatment at 525o C for 8 hours and (c) 525oC for 24 hours was evaluated to observe the relationship between corrosion rate and grain size of the Mg sample. Similarly, commercial purity Mg, Mg-RE alloys ZE10Aand EV31A were evaluated in as received condition for potential application as anode for Mg-air battery. Anodic and cathodic polarization studies were performed in 0.6M NaCl, 0.5M NaNO3 and 0.1 M Na2SO4 electrolytes. Hydrogen evolved during anodic polarization was measured at different potentials and electrochemical impedance spectroscopy was carried out at open circuit conditions. The discharge behavior of lab made battery was also tested to compare the performance of the alloys in the battery.
There is no unequivocal relation is available to describe the effect of grain size on the corrosion behavior. Several reports point out improvements in the corrosion resistance of the Mg alloys upon grain refinement. A decrease in the corrosion resistance has also been observed with the decrease in the grain size for different materials such as pure Mg, Al alloy, and Ni- base alloy. Ralston et al. correlated the improvement in the corrosion resistance by the grain refinement to the ability of the metal to form a protective passive layer by improved ionic conduction of the grain boundaries. The refinement of grain is generally achieved by severe plastic deformation processes such as such as surface mechanical attrition, equal channel angular pressing (ECAP), and friction stir processing. Reduction in grain size is also associated with other microstructural changes such as increased dislocation density, point defects, and creation of internal stresses during these mechanical processing techniques. Therefore, the observed changes in the corrosion behavior due to variations in the grain size cannot solely be attributed to the grain boundary structures. In order to isolate the effect of grain size, other parameters such as dislocation density, defect structures, phase content, secondary phases and their composition, shape, size, and volume fraction need to be maintained at the same level and only the grain size should vary. In this work, the effect of grain size on the corrosion of ZE10A has been investigated without subjecting the material to plastic deformation.
Magnesium exhibits a phenomenon known as negative differential effect which is characterized by the increase in the rate of hydrogen evolution with increasing anodic potential. The strategy for minimizing the NDE effect would be to decrease the impurity elements to very low levels and add elements such as arsenic that will poison the hydrogen recombination and minimize the hydrogen evolution. Another strategy to minimize hydrogen evolution is to design new electrolytes. In a recent study, Richey et al showed that hydrogen evolution was the highest in 0.5 M NaCl solution and the lowest in 0.5 M NaNO3 solution. In the research, anodically polarized Mg surface was found to comprise of Mg(OH)2/MgO based bilayer film which presented a peculiar problem, as ideally a hydroxide film would slow down the kinetics of the electrochemical reaction, but it seemed to enhance the kinetics of hydrogen evolution reaction. Mg surface was found to be proportional to the rate of film growth. Many studies have shown that Mg undergoes rapid dissolution accompanied by Mg(OH)2 layer forming spontaneously upon the surface which could enhance parasitic corrosion in primary Mg batteries and affect the efficiency of Mg and decrease the life span of Mg alloys used in light weight applications. The volumetric HE collection method is used for calculation of corrosion rate during anodic polarization. The volume of the hydrogen collected is used to calculate the reduced Columbic efficiency of the battery based on the relation:
Q total= QH2+Qi net anodic.
The main aim of the work is to investigate the polarization behavior Mg-rare earth (RE) alloys such as ZE10A (Elektron 717) and EV31A (Elektron 21) as potential anodes for Mg-air battery, and compare their behavior with that of commercial purity Mg. The rationale behind selecting these alloys is to understand the role of Zn and Nd alloying additions on the anodic activation of the magnesium surface for battery application. Furthermore, electrochemical characterization was carried out in three different electrolytes consisting of NaCl, Na2SO4 and NaNO3 salts to understand the role of electrolyte on the NDE of the Mg-RE alloys.